With recent advances in the digitization of medical data, various types of medical images are archived as digital images, and doctors have increasing chances of performing image diagnosis upon displaying digital images on displays. In general, medical images are stored in formats complying with DICOM (Digital Imaging and Communications in Medicine) international standards. Medical images include CT (Computed Tomography) images and MRI (Magnetic Resonance Imaging) images. Each type of image includes a large amount of three-dimensional image data constituted by many tomographic images. A medical image display apparatus is used to efficiently and meticulously observe such a large amount of image data.
A conventional medical image display apparatus has, as display functions for tomographic images, a function of moving a tomographic position, a function of enlarging/reducing an overall image, a function of translating a display position, a gray level transform function, and the like. In this case, the gray level transform function is a function of transforming each pixel value of a gray scale according to a predetermined rule. In general, a CT image is expressed by a 12-bit gray level (4,096 gray levels) HU (Hounsfield Unit) value per pixel, which is stored as a pixel value. When a doctor observes a CT image, a medical image display apparatus displays the CT image upon transforming the HU values into the number of gray levels suitable for display. In this transform, 12-bit gray level pixel values are transformed into 8-bit gray levels by using, for example, two gray level transform parameters, that is, WC (Window Center) and WW (Window Width), defined by DICOM standards. That is, the image (display image) having simultaneously undergone a change in the number of gray levels and a change in gray level distribution is displayed on a display. This makes it easy to see the difference in HU value (gray level difference) between a specific organ and a tissue. For an image other than a CT image (for example, an MRI image, PET image, or ultrasound image), gray level transform similar to that described above is performed to make it easy to see the differences between pixel values (gray level differences).
Although the above gray level transform function makes it easy to see image gray level differences, this rather makes it difficult to see original pixel values (HU values of a CT image) (before changes in the number of gray levels and gray level distribution). For this reason, some computer software has a function of displaying the original pixel value of each pixel in numerical string. For example, as shown in FIG. 6, when the user designates a target region by drawing a rectangle (graphic 52), the software obtains an image in the target region, enlarges and displays the image in another window 62 at a high magnification, and displays the original pixel value (HU value) of each pixel on the enlarged image.
Computer software used in general has an enlarged display function called loupe function for a frame in a rectangular region, as shown in FIG. 7. With the loupe function, when the user moves the cursor, the software generates a frame by enlarging a frame in a rectangular region with a predetermined size centered on the cursor position at a predetermined magnification, and displays the frame (graphic 63). In this case, the software displays the enlarged frame such that its center coincides with the cursor position (http://www.mathworks.co.jp/jp/help/images/ref/impixelregion.html).
When using a medical image display apparatus like that described above, there are demands to see a display image before enlargement, that is, an overall image, and simultaneously see an enlarged image in a target region and each pixel value in the target region. However, in the above pixel value display, since a display image and each pixel value are respectively displayed in windows at different positions, it is difficult for the user to simultaneously see them. On the other hand, when using the loupe function, although an enlarged frame is displayed at the cursor position, the frame before enlargement is hidden in the region where the enlarged frame is displayed. Even if, therefore, the pixel value display function is combined with the loupe function, the above problem cannot be solved.